Phenolic resinous compositions were produced for the first time at the beginning of the 20th century after A. von Baeyer has discovered the reaction between phenols and aldehydes in 1872.
In recent years, there has been an increased interest in phenolic polymers which can be formed into cellular materials more commonly referred to as phenolic foams. Since phenolic foams were mainly viewed as good insulating products possessing excellent flame resistant properties, a large part of the research conducted in the field over the past decades has focused on ways to improve these properties.
Prior to 1968, it was generally taught to use thermosetting phenolic resin foam structures that could be prepared by reacting an acid catalyst with a heat hardenable phenol-aldehyde one step resin, commonly called "resol".
Resols and novolaks are products of the reaction of phenols with formaldehyde and these resins have been used for a variety of applications. Although resols are normally made with formaldehyde, other aldehydes are often used in the event certain desired characteristics are required for the resulting phenolic resin product. Resol can be defined as synthetic resin produced from a phenol and an aldehyde. The molecule contains reactive methylol or substituted methylol groups.
Heating causes the reactive resol molecules to condense together to form larger molecules. This result is achieved without the addition of any substances containing reactive methylene or substituted methylene groups. As for novolak, it is a soluble fusible synthetic resin also produced from a phenol and an aldehyde. However, the novolak molecule does not contain any reactive methylol or substituted methylol groups and is therefore incapable of condensing with other novolak molecules on heating without the addition of hardening agents.
The reaction of an acidic catalyst with a resol resin being exothermic, this creates sufficient heat to convert the water of condensation and any water initially present in the mixture to steam. This steam, which is usually uniformly distributed throughout the resin, foams the reacting resin into a frothy mass and because of the rapid exotherm, the resin converts quickly into an infusible condition before the froth can collapse to any significant extent because of the condensation of the steam. The foam resulting from this type of process was referred to as an "open cell" foam. This type of foam was believed to be undesirable since it did not possess uniform insulating properties.
In an attempt to develop a phenolic foam having improved insulation and flame retarding properties, D'Alessandro produced an improved phenolic resin foam using polyhalogenated saturated fluorocarbons to produce a fine uniform closed cell structure in the phenolic foam. The retention of fluorocarbons in the closed cell structure lead to a higher insulation value.
Since the D'Alessandro patent, it has been established that fluorocarbon blowing agents contribute in deteriorating the ozone layer around the surface of the earth. Therefore, other suitable commercial alternatives need to be sought. Also, the brittleness of the rigid phenolic foams closed or open cell was a major inconvenient.
In 1975, Ernest K. Moss in U.S. Pat. No. 3,876,620 attempted to solve this problem by introducing a phenolic polymer exhibiting a friability of less than 15% when formed into a cellular product. This was achieved by using o-cresol in the phenolic composition.
In 1979, Ernest K. Moss and John Beale in U.S. Pat. No. 4,133,931 further improved on the 1975 Moss invention by providing an improved closed cell phenolic-resin foam material which exhibited low thermal conductivity without adversely affecting friability, compressive strength and the usual low flammability characteristics of this type of material. In this case, good results were achieved by using a branched non-ionic surfactant but now undesirable fluorocarbons were used as blowing agents.
In 1981, Gusmer, in U.S. Pat. No. 4,303,758, disclosed a novel technique through which a closed cell phenol-aldehyde foam product could be obtained. This method involved a frothing and curing technique that was designed by the inventor.
Although phenolic foams have been known and available for many years, they did not penetrate substantially the thermal insulation market until only recently. Two major drawbacks made the commercialization of closed cell phenolic foam in the thermal insulation market difficult. First, closed cell cellular foams tend to loose their insulating properties over time because the brittleness of their structure inevitably leads to he partial breakdown of the closed cell network, thereby releasing the fluorocarbons entrapped in these cells. The entrapped fluorocarbons contribute to enhance the thermal insulation properties of the foam. Also, since all phenolic foams described so far in the prior art are rigid, their compressive strength is usually quite limited and most often unsuitable for normal handling unless extreme care is taken in manipulating the product.
As mentioned earlier, the general composition and method for preparing phenolic foams are well known. Generally speaking, a foamable phenolic resole composition is prepared by admixing a liquid phenolic resole, a blowing agent, a surfactant, and optical additives as well as an acid curing agent into a substantially uniform composition. The curing catalyst is usually added in amounts sufficient to initiate the highly exothermic curing reaction. This step, usually referred to as the foaming step, is most often conducted or performed in a cavity mold or a continuous laminating machine.
When it is desired to manufacture phenolic foam insulation boards, the various ingredients are mixed until a substantially uniform composition is obtained. This composition is then applied onto a protective covering such as cardboard to which it will initially adhere. The foam is then covered with an other protective covering such as cardboard. The covered foaming composition is then passed into a double-belt press-type apparatus where the curing exotherm continues to vaporize and expend the blowing agent, thereby forming the composition as it is cured.
All the processes mentioned above lead to rigid phenolic foams exhibiting varying degrees of compressive strength properties. Therefore, the closed cell phenolic foams designed so far face two major problems. First, they are allowed to contain entrapped fluorocarbons for a somewhat higher insulation value but which are detrimental to the earth's atmosphere. Secondly, in many prior art phenolic foams, when cell walls are subjected to high temperatures or to external pressure, the cell walls cannot resist to these pressures and crack. Obviously, cracking of the cell walls allows the fluorocarbon blowing agents to leak out during curing or over time which consequently decreases the insulation properties of the product.
Mendelsohn et al. and Smith, respectively in U.S. Pat. Nos. 4,107,107 and 4,350,776, designed phenolic foams that could be used as high compressive strength, non-flammable materials while possessing low friability properties. In the case in U.S. Pat. No. 4,107,107, the result was achieved by using a process requiring both a dual surfactant and a dual acid system. Still, the phenolic foam described in U.S. Pat. No. 4,107,107 is a rigid foam system with or without entrapped fluorocarbons.
In the case of U.S. Pat. No. 4,350,776, the inventor used a furfuryl-alcohol polymer to obtain a low friability and flame resistant thermosetting foam. This foam is also a rigid phenolic foam product having entrapped therein suitable blowing agents such as fluorinated compounds.
In both cases, however, the final product still has a rigid form and even though compression properties have been improved, the foam is still friable.
Thus, substantially rigid phenolic foams have been known for many years. The rigidity and friability of these foams and the inherent lack of elastic deformation characteristics are such that these foams can be easily damaged when handled or when they are submitted to any type of external pressure. Most of these foams become more friable over time. As a result, these foams cannot be utilized in many applications where their good insulating and fire resistant properties would otherwise make their use preferable. Such applications include cushioning and insulation where flexibility and elasticity are required to reduce damage that could occur by friction fitting, impact, loading or lifting. In these applications, the known rigid phenolic foams crush under load and remain permanently deformed. Consequently, although the rigid close cell or substantially close cell phenolic foams containing entrapped fluorocarbon blowing agents have excellent insulation properties, their use has been found to be fairly limited.
In the case of open cell phenolic foams, it was thought for a long time that these foams were to be used only in conditions where easy friablity of the product was desired. For example, Smithers in U.S. Pat. No. 2,753,277 describes a material employed for floral supporting means, the material being made of a phenol-formaldehyde foam to which a foaming agent and a catalyst are added. Similar products are described in U.S. Pat. No. 3,049,444 issued to A. M. Palombo who disclosed a cellular synthetic material impregnated with a wetting agent. The synthetic material is a phenol-formaldehyde foam.
In these documents as well as in other U.S. Pat. No. documents such as U.S. Pat. Nos. 3,287,104, 3,697,457 and 4,225,679, phenolic resoles are mixed with a surfactant, a wetting agent, a dye and a blowing agent. The resulting mixture is then reacted with a catalyst, usually an acidic catalyst and allowed to expand and cure to a rigid, hard and highly friable foam, capable of absorbing water and allowing the flower stems to enter the wet foam by applying only a slight pressure.
Thus, none of the open or close cell phenolic foams disclosed in the prior art possess the desired flexibility properties. Furthermore, in both cases, it seemed so far to have been impossible to obtain suitable stable flexibility of the foam while maintaining acceptable insulating and fire resistant properties.